Experimental Study On Road Performance Of Metakaolin-steel Slag Composite Modified Expansive Soil

Expansive soil is widely distributed in China,resulting in great engineering hazards and economic losses.Improving the engineering properties of expansive soil has been a hot issue in geotechnical engineering.At the same time,as a worldwide steel production and steel slag output power,compared with the United States,Germany,Japan and other developed countries,steel slag conversion utilization rate in China is low.The accumulation of steel slag leads to land occupation and river sedimentation,which brings serious environmental pressure.Therefore,it is urgent to carry out the research on the utilization of steel slag resources.In order to solve this problem,this paper relies on the national natural science fund project(41672306),using metakaolin “pozzolanic activity” to stimulate steel slag hydration reaction,forming composite materials in alkaline environment to improve the road performance of expansive soil.The specific research contents and conclusions are as follows:(1)The free expansion rate,shrinkage rate,load expansion rate and other tests were carried out to obtain the expansion and shrinkage characteristics of metakaolin-steel slag composite modified expansive soil.The above three kinds of tests were added 3 %lime to provide alkaline environment.At the same time,metakaolin(0 – 2 % – 4 % –6 %)and steel slag(0 – 5 % – 10 % – 15 %)were mixed in the same gradient and controlled by a single variable.The test results of free expansion rate show that when steel slag is added more than 10 % and metakaolin is added more than 4 %,the free expansion rate of soil samples at 7d and above 7d curing ages decreases to about 5 %.The test results of load expansion rate show that when steel slag is added more than 10 %and metakaolin is added more than 4 %,the load expansion rate of soil samples above 7d and 7 d decrease to about 0.(2)The unconfined compressive strength test and compression consolidation test were carried out to obtain the variation law of mechanical properties of modified expansive soil with metakaolin-steel slag composite.In both tests,3 % lime was added to provide alkaline environment.At the same time,metakaolin(0 – 2 % – 4 % – 6 %)and steel slag(0 – 5 % – 10 % – 15 %)were mixed in the same gradient and controlled by a single variable.From the results of unconfined compressive strength test,it can be seen that when more than 10 % and metakaolin was added to 1/2 – 1/3 of steel slag,the unconfined compressive strength of soil samples at 7 d and above 7 d curing ages increased to more than 2 MPa.By comparing the strength ratios of soil samples at 7 d and 28 d curing ages when steel slag was mixed with 15 %,it was found that the content of metakaolin was positively correlated with the ratio,which verified the conjecture that metakaolin played a positive role in the early hydration activity of steel slag.The results of compression consolidation test show that when steel slag is added more than 10 %and metakaolin is added more than 4 %,the compression modulus of soil samples increases to more than 20 MPa at 7 d and 7 d curing ages.(3)XRD,SEM,MIP and other microscopic tests were carried out to obtain the change law of microstructure characteristics of metakaolin-steel slag composite modified expansive soil.In both tests,3 % lime was added to provide alkaline environment.At the same time,the typical dosage of metakaolin and steel slag was selected for microscopic mechanism analysis combined with the expansion-shrinkage characteristic test and mechanical property test.The test results verify that the main hydration products widely distributed in the expansive soil modified by metakaolin-steel slag composite are C-S-H,C-A-S-H,Aft,etc.At the same time,the surface of the soil particles was observed by scanning electron microscopy,and it was found that the hydration products formed wrapping or cementation on the soil particles,which further explained the improvement mechanism of the swelling and shrinkage characteristics and mechanical properties of the soil samples,and provided a theoretical basis and basis for metakaolin and steel slag in improving expansive soil under alkaline environment.